The present invention relates to a system and method of controlling an automatic transmission for a vehicle, and more particularly, to changeover control for releasing engaging means in the engaged state and engaging another engaging means in the released state at shift operation.
Typically, the automatic transmission achieves multiple speeds by changing the power transfer path of a gear train through engagement and release of frictional engaging means in such a manner as to mutually engage speed-change elements with a planetary gear by a clutch or fixing them to a transmission casing by a brake. In the automatic transmission, at downshift between two adjacent speeds, for example, from third speed to second speed, changeover is carried out by releasing the first engaging means to be engaged for achieving third speed while engaging the second engaging means to be engaged for achieving second speed.
In such shift operation, the releasing and engaging timing of the engaging means is of importance. If engagement lags behind release, engine idle revving occurs, whereas if engagement fairly leads release, tie-up between the two engaging means occurs, leading to a greater shift shock.
JP-A 9-49566 describes a technique for properly achieving the changeover timing of the two engaging means. In this reference, an accumulator back-pressure map is set to correspond to the vehicle speed and the throttle acceleration. The releasing pressure and the engaging pressure of the two engaging means are adjusted by using the vehicle speed, the throttle acceleration, and preprogrammed values, thus achieving optimized changeover timing.
With the above technique, however, changeover control is carried out in accordance with the throttle acceleration, having the following problem to be solved.
FIG. 5 shows a shift line 3-2 for second speed and third speed. When a throttle opening TH crosses the shift line 3-2 from bottom to top, downshift is made from third speed to second speed. FIG. 5 shows two examples for a change in throttle opening, which cross the shift line 3-2, one being a case of changing the throttle opening TH from Ab to Aa and another being a case changing the throttle opening TH from Bb to Ba. Assuming that the two examples are of the same variation TH of the throttle opening TH.
When changing the throttle opening TH at the same speed, control is carried out, in either case, to decrease the release-side accumulator back pressure in a short time due to the same throttle acceleration.
When the throttle opening TH is immediately before crossing the shift line 3-2, and thus has a small value, the changeover timing can be optimized. However, when the throttle opening TH has a large value, i.e. when the throttle opening TH varies from Bb to Ba, input torque from the engine to the automatic transmission is increased in advance. In this state, if the release-side accumulator back pressure or releasing pressure is reduced in a short time, the first engaging means engaged before engagement of the second engaging means to be engaged is changed to the torque non-transfer state in a short time, leading to not only occurrence of drawing that the vehicle acceleration lowers once to a value close to zero G as illustrated in FIG. 5, but also occurrence of engine idle revving.